Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

From: Oscar Salvador <osalvador@suse.de>
To: Muchun Song <songmuchun@bytedance.com>
Cc: "Michal Hocko" <mhocko@suse.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
	dave.hansen@linux.intel.com, luto@kernel.org,
	"Peter Zijlstra" <peterz@infradead.org>,
	viro@zeniv.linux.org.uk,
	"Andrew Morton" <akpm@linux-foundation.org>,
	paulmck@kernel.org, mchehab+huawei@kernel.org,
	pawan.kumar.gupta@linux.intel.com,
	"Randy Dunlap" <rdunlap@infradead.org>,
	oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
	"Mina Almasry" <almasrymina@google.com>,
	"David Rientjes" <rientjes@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
	"David Hildenbrand" <david@redhat.com>,
	"HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	"Xiongchun duan" <duanxiongchun@bytedance.com>,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	"Linux Memory Management List" <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
Date: Mon, 22 Feb 2021 11:50:56 +0100	[thread overview]
Message-ID: <20210222105051.GA23063@linux> (raw)
In-Reply-To: <CAMZfGtWt3uYcCv5htRkOncJnh=4eiGXzpKsV7-Gj40m-BXcUrw@mail.gmail.com>

On Mon, Feb 22, 2021 at 06:31:12PM +0800, Muchun Song wrote:
> On Mon, Feb 22, 2021 at 5:25 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Sat 20-02-21 12:20:36, Muchun Song wrote:
> > > On Fri, Feb 19, 2021 at 10:12 PM Michal Hocko <mhocko@suse.com> wrote:
> > [...]
> > > > What about hugetlb page poisoning on HW failure (resp. soft offlining)?
> > >
> > > If the HW poisoned hugetlb page failed to be dissolved, the page
> > > will go back to the free list with PG_HWPoison set. But the page
> > > will not be used, because we will check whether the page is HW
> > > poisoned when it is dequeued from the free list. If so, we will skip
> > > this page.

Not really. If the huge page is dissolved, we will take the page out of the
the freelist. See take_page_off_buddy in memory_failure_hugetlb.

In an ideal world, we should inspect that page in free_pages_prepare(),
remove the HPWpoisoned page and process the others, without letting that
page hit Buddy.
And not only for hugetlb, but for any higher order page.
See how memory_failure() happily disengage itself when it finds a higher
order page.
It does it because we have the premise that once that page hits Buddy,
it will stay there as the check_new_page guards us.
But this has been proofed to be quite a weak measure, as compaction does
not performs such a check, and so the page can sneak in.

I fixed that for soft-offline, and for memory-failure in some cases, but more
needs to be done and is it in my TODO list.

> > Can this lead to an under provisioned pool then? Or is there a new
> > hugetlb allocated to replace the poisoned one?
> 
> Actually, no page will be allocated. Your concern is right. But without
> this patch, the result does not change. e.g. The HW poisoned page
> can fail to be dissolved when h->free_huge_pages is equal to
> h->resv_huge_pages. But no one seems to have reported this issue so
> far. Maybe this behavior needs improvement in the feature.

Yes, something to improve.
I shall have a look.

-- 
Oscar Salvador
SUSE L3